Antennas are widely used for communication purposes and for radar or other sensing use. In general, an antenna must be adapted for the intended use. In some applications, high antenna gain is desired. High antenna gains are associated with high directivity, which in turn arises from a large radiating aperture. A common method for achieving a large radiating aperture is by the use of parabolic reflectors fed by a feed arrangement located at the focus of the parabolic reflector. Parabolic reflector type antennas can be very effective, but for certain purposes may present too much of a wind load, and for scanning use may have too much inertia to achieve the desired scanning acceleration. Also, reflector antennas in general suffer from the problem of aperture blockage attributable to the support structure required to support the feed antenna, and the feed antenna itself, which may adversely affect the field distribution over the surface of the reflector, and thereby perturb the far-field radiation pattern.
Modern communication and sensing systems find increasing use for antenna arrays. An antenna array includes an array or battery of usually-identical antennas or elements, each of which ordinarily has lower gain than the array antenna as a whole. The arrayed antenna elements are fed with an amplitude and phase distribution which establishes the far-field "radiation" pattern or beam. Since the phase and power applied to each antenna element of an array antenna can be individually controlled, the direction and characteristics of the beam can be controlled by control of the distribution of power (signal amplitude or gain) and phase over the antenna aperture. A salient advantage of an array antenna is the ability to scan the beam or beams electronically, without physically moving the mass of a reflector, or for that matter any mass whatever.
Many problems attend the use of array antennas. While a reflector is not necessary (although one may be used, if desired), achieving high gain still requires a large effective radiating aperture. The far-field radiation pattern of an array antenna is the product of the radiation pattern of one of the antenna elements, multiplied by the radiation pattern of a corresponding array of isotropic sources (sources which radiate uniformly in all directions), or in other words the product of the radiation pattern of an individual antenna element multiplied by the array factor. Thus, achieving high gain in an array antenna may require an array factor giving high gain, an individual antenna element having high gain, or both. The array factor can be increased to a certain extent by increasing the distance between individual element, but when the inter-element spacing becomes large, grating lobes may degrade the desired radiation pattern. Thus, achieving high gain in an array antenna may depend upon use of relatively high-gain antenna elements.
Those skilled in the art also know that one of the salient characteristics of an antenna is its field polarization. There are two general classes of field polarization, one of which is linear, and the other of which is circular. In the case of linear polarization, the electric field vector of the radiated beam appears, at a given location far from the antenna, as a line, which may be oriented in any desired direction, as for example vertically or horizontally. In the case of circular polarization, on the other hand, the electric field vector rotates in a plane orthogonal to the direction of propagation at a rate related to the frequency of the propagating wave. It should be noted that the term "circular" polarization refers to a theoretical condition which is approached only on rare occasions, and the term "circular" is often applied to imperfect circular polarization which would more properly be termed "elliptical".
When circular polarization is desired in the context of an array antenna, a circularly polarized antenna element is often used. U.S. Pat. No. 5,258,771, issued Nov. 2, 1993 in the name of Praba, describes an array antenna in which circular polarization is achieved by the use of axial-mode helical antennas. In the Praba arrangement, the axial mode helical antenna elements themselves have relatively high gain. For some applications, the three-dimensional axial-mode helices may be too large, and may, when arrayed, define an antenna which exceeds the available volume for its installation. For example, in many applications, such as for spacecraft or aircraft, small volume and light weight of an antenna array are extremely important.
Planar spiral antennas are known. Such planar spiral antennas are generally lightweight, and have small volume. Planar spiral antennas provide nominally circular polarization, but have other disadvantages. Some planar spiral antennas require installation in a "cup" to reduce radiation in the plane of the antenna. Many planar spiral antennas require a "balun" or balanced-to-unbalanced transformer. Baluns are undesirable, because of their bandwidth and or power limitations, and because they may require axial depth for effective operation.